1. Field of the Invention
The invention relates generally to signal receivers and more particularly to a receiver for bandspread signals.
2. Description of the Prior Art
In the field of signal processing, bandspread signals are signals in which a high frequency carrier is encoded with a pseudo-random code (hereinafter PRC). The PRC which is a digital spectrum-spreading code is modulated by a modulation signal and is employed for data transmission. Frequently, the modulation signal is also a digital signal. In this case, a second digital signal is thus superimposed on the first digital signal (e.g. the PRC) for data transmission. Generally, the clock rates of the first digital signal (PRC) and the second digital signal (modulation signal) are significantly different.
The PRC is employed by the transmitter for bandspreading signals but is also stored in the receiver. Moreover, the receiver also contains a clock pulse generator which producers a clock pulse signal for controlling the read-out of the PRC from a memory It is necesary for the phase of the clock pulse signal to be controlled so as to produce temporal coincidence between the PRC stored in the receiver and the PRC of the received signals. The range of the transmitter station from the receiver can be determined on the basis of the phase shift between the PRC of the received signals and the PRC stored in the receiver.
Examples of systems which employ bandspread signals are the Joint Tactical Information Distribution System (JTIDS) and the Global Positioning System (GPS). The GPS is described in the publication "Navigation: Journal of the Institute of Navigation", Vol. 25, No. 2, Summer 1978, pages 121 to 146. The article is entitled "GPS Signal Structure and Performance Characteristics", by J. J. Spilker, Jr. Receivers for bandspread signals are described on pages 139 to 146 of the above-cited GPS publication. In the prior art GPS receiver, the phase of the stored PRC is controlled with a .pi.-dither control loop or an early/late control loop. Also, a costas loop is provided in which I and Q components of a modulated carrier signal are formed following suitable signal processing involving conversion to an intermediate frequency (IF) value. Further, the costas loop permits the modulated signal to be recovered. However, a problem exists in the prior art in that the implementation of such receivers is very complicated and costly.